Modern communication networks typically link many different types of mobile and/or stationary communication terminals. Among the communication networks are the well known cellular phone backhaul networks and the less familiar smart grid networks, which operate to control and distribute energy. The networks may link communication terminals, such as by way of example, cellular phones, computers, and communication enabled industrial plant equipment, and provides the terminals with an ever increasing menu of voice, video, and data communication services.
The networks operate to transport information from one to another of their respective terminals using signals containing information relevant to the services that the networks provide. In propagating from a source terminal to a destination terminal of a given communication network, the signals generally propagate through a plurality of network nodes. At each node the signals are received and then, optionally after processing in the node, are forwarded toward their destination. Typically, the networks are packet switched networks (PSNs) and information propagated by the networks are packaged in packets configured in accordance with a suitable packet technology such as multiprotocol label switching (MPLS), internet protocol (IP), and/or Ethernet.
Various services, such as cellular telephony and data, when transported over PSN networks, require for their proper operation that devices in the PSN network be synchronized to highly accurate timing information. Timing information comprises a highly accurate reference frequency and/or time of day (ToD). ToD defines a current year, month, day, hour, minute, second, and fractions of a second, referenced to some standard, such as International Atomic Time (TAI) or Universal Coordinated Time (UTC). A network device in a PSN network may receive timing information from various sources and in accordance with various synchronization procedures. For example, a network device in a PSN may receive timing information directly from Global Navigation Satellite System (GNSS) transmissions, such as the US Global Positioning System (GPS), the Russian GLONAS, or the Chinese Beidou satellite transmissions, or by participating in a PSN “timing over packet” procedure, executed in accordance with a suitable packet timing distribution protocol, whereby a “packet slave clock” is synchronized to a “packet master clock”.
Timing over packet is generally provided by a system of clocks comprising a single reference clock referred to as a “packet grand master clock” that communicates and synchronizes time with at least one packet slave clock. The packet grand master clock receives a frequency reference and a time of day (ToD) reference from a highly accurate reference clock, such as a Primary Reference Time Clock (PRTC). A PRTC may provide the frequency reference as an isochronous train of pulses, referred to as “clock pulses”, characterized by an accurate and stable pulse repetition frequency (e.g., 10 MHz). The PRTC may provide signals for determining ToD as a sequence of narrow pulses having a repetition rate at one pulse per second (1-PPS), with each pulse accompanied by a time code that associates the pulse with a year, month, day, hour, minute, and second, referenced to a standard such as TAI or UTC. The PRTC may comprise a highly stable Cesium or Rubidium atomic clock in order to maintain highly accurate frequency. Additionally or alternatively, it may comprise a GNSS radio receiver in order to receive accurate ToD information from GNSS satellite transmissions. The reference frequency provided by a PRTC is generally required to be accurate to better than 1 part in 1011 and the ToD accurate to ±100 ns (nanosecond) relative to UTC. The packet master clock repeatedly, and usually at regular time intervals, synchronizes each packet slave clock responsive to the reference frequency and ToD that it receives from the PRTC, in a process referred to as “timing distribution”.
Timing distribution involves a packet master clock and packet slave clock exchanging a sequence of timing packets configured in accordance with communication protocols of the network. Commonly used protocols are Network Time Protocol (NTP), versions of which are defined in RFC-1305 and RFC-5905, and Precision Time Protocol (PTP), versions of which are defined in IEEE-1588-2002 and IEEE-1588-2008 (often called 1588-v2). The timing packets comprise timing information, such as “timestamps”, which the packet slave clock records, and which define times at which the timing packets egress and/or ingress the master clock and/or the slave clock. Upon completion of a transaction, the packet slave clock has a record comprising a set of timestamps that it uses to synchronize itself to the packet master clock.
Typically, a packet master clock of a PSN distributes timing to a plurality of packet slave clocks, each of which is located at a different node of a plurality of nodes of the network. To provide a satisfactory degree of reliability, the packet master clock may comprise redundant components, such as a back-up power supply and redundant packet transmission and reception circuitry. Due to the expense of the PRTC and packet master clock, and in order to ensure consistency, the network generally comprises a single packet master clock connected to the physical layer of the network at a central location in the network.
Packets propagating between the same two nodes in a PSN may experience different transit times, referred to as propagation delays, in propagating between the two nodes. Variations in propagation delay (propagation delay variation—PDV) may result from variations in queuing delays in network elements along paths that packets travel between the nodes and/or from packets traveling along different paths between the nodes. A difference between a propagation delay from a first node to a second node, and a propagation delay from the second node to the first node is referred to as delay asymmetry. Consistent delay asymmetry may result from the path from the second node to the first node not coinciding with the path from the first node to the second. In addition to causes of delay asymmetry and PDV resulting from the logical structure and operation of the PSN network noted above, delay asymmetry and PDV may also be generated by changes in physical hardware comprised in the physical layer supporting the PSN. Such changes may for example comprise changes in physical connectors, fiber and/or copper cabling, and/or interface circuitry. For example, delay asymmetries and PDVs of hundreds of nanoseconds (depending on cable length) may be caused by environmental changes affecting cabling and cable connectors. Overall, delay asymmetries and PDVs can be on the order of 10 s or even 100 s of milliseconds.
Delay asymmetries and PDVs inherent in a PSN network degrade the quality of timing recovered by a packet slave clock using timing over packet protocols. Furthermore, as the number of nodes in a PSN network increases and a number of alternative communication paths between a master clock and slave clocks increases, the difficulty, expense, and bandwidth overhead incurred in distributing timing to the network clocks increases.